This invention relates to electrode probes for measuring corrosion currents of metals immersed in ionically conducting liquids.
Corrosion current measurements made in accordance with my U.S. Pat. No. 3,694,324 require the use of at least three electrodes. In one alternative, two duplicated measured electrodes and one electrode operated as an anode are required. In another alternative, one measured, one reference, and one opposed electrode are required.
In practice, each electrode with its lead wire is held in an electrical insulator, the combination being termed an electrode assembly. A plurality of electrode assemblies are positioned and held in selected spacial relationship by securing them to an electrode holder. The electrode holder can vary in form according to how it is in turn held in selected spacial relationship to the liquid corrosive. The simplest form of electrode holder is one supported by the top edges of the container holding the liquid corrosive. More complicated forms of electrode holders are used for holding electrodes in tanks and pipes, and the combination of electrodes and electrode holder has been termed an electrode probe by the manufacturer and by users in the field.
An early form of electrode probe is illustrated in FIG. 12 of said above patent. A plurality of cylindrical electrodes of the corroding metal are secured at one end in selected spaced relationship and with their major axes parallel, by an insulator, such as an epoxy plastic, that is cast into a metal cylinder, frequently of stainless steel.
This form of probe has proven to be very useful in the control of corrosion occurring in the field, through the monitoring of corrosion rate measurements made at selected spaced intervals of time. Since such monitoring generally requires only relative corrosion current measurement, it is possible to reuse the probe occasionally after cleaning the electrodes.
This probe has also been elaborated in the form of removable electrodes, which permits weighing of the electrodes before and after the observed duration of the corrosion, and which enables repeated use of the electrode holder to economic advantage. The electrodes are screwed into the holder, and the bolt threads can alternatively be on the end of the holder. The end of the electrode is sealed to the insulator by means of an elastic washer fitting over the bolt member.
A higher order of measurement performance is required from the probe when accurate corrosion rate measurements are applied to research and development applications evaluating the single and combined effects of factors of corrosion system performance, such as metal composition, corrosive liquid composition, corrosive environment such as temperature and flow rate, the use of inhibitors, and the possible presence of accelerators.
One limitation to the above form of probe is the metal cylinder into which the insulator is cast. With the finite volume of corrosive liquid generally used in the laboratory, the additional metal area introduced by the cylinder can alter the performance of the corrosive with the passage of time. If the composition of the metal cylinder differs from that of the electrodes, its corrosion products can cause a further disturbance.
Other limitations originate from the use of a single insulator body of diameter several times that of a single electrode, with the electrode surfaces terminating into the cross-sectional area of the insulator body. When the corrosive solution flows past the electrodes, the uniformity of flow paths is substantially disturbed by the insulator body.
Uniformity of current flow is disturbed by the combined effects of an insulator body shield at the top of the electrodes and no shield at the bottom tips of the electrodes. Conditions favoring uniform current flow to and from electrode surfaces are essential to producing corrosion current-time relationships that accurately measure metal losses when correlated with weighed metal losses. These conditions become more critical as the ionic conductor resistivity increases into the region of make-up water and passes into the range of distilled water.
The washer of the removable electrode probe that seals the electrode to the insulator is seated on a shoulder machined on the electrode in a plane perpendicular to the major axis of the electrode. Exposed electrode surface terminates at a 90.degree. angle to the washer, which favors the development of crevice corrosion along the electrode surface in contact with said washer. Such corrosion is objectionable because it is shielded from corrosion current measurement, and because it diminishes the contact area relied upon to keep the electrode in a tightly screwed-in position in the presence of vibration.
The above limitations recognized in these probes are in part corrected in this invention through mechanical design, but the substantial elimination of crevice corrosion did not initially appear to be possible.
Crevice corrosion occurs in the presence of an accelerator such as dissolved oxygen. When a small area of the total exposed metal surface is shielded from the cathodic depolarizing action of the dissolved oxygen, it operates as an anode area with corrosion rate accelerated through short-circuit coupling to the large exposed surface area undergoing cathodic depolarization. The finding that crevice corrosion was serious when the electrode surface was sealed to insulator surface with beeswax, was understandable through recognition that anodic corrosion products could easily deform the wax and increase the anodic area. Sealing the electrode surface to the insulator surface with an epoxy resin cured in position with its admixed hardener, to form one of the strongest known adhesive bonds, diminished but did not substantially eliminate the crevice corrosion.
The concept of a seal in the form of an elastic material pressed to the metal surface through tension forces, was tried in the form of rubber tubing stretched over cylindrical electrode surface. This concept became workable when the interface between the shield and metal was made water-repellent by a coating of Vaseline. Later, a silicone grease, inert to rubber and noted for its strong water repellency and good temperature resistance produced more reliable results, and promotes an easily removable form of shielding.